One of medicine's primary objectives is to trick the body into doing something it doesn't want to do. We try to convince our immune systems to attack cancer cells (our immune systems don't normally attack our own bodies), we try to convince neurons to regrow (another unnatural phenomenon), and we try to convince the body to accept foreign bits, such as someone else's kidney or a fake bone. In order to accomplish this, we try to make parts of our bodies we don't want, such as cancers, look foreign. We try to make foreign bits that we do want, such as orthopaedic implants, look natural. Nanotechnology, as you might have guessed, can help us do just that.
At the nanoscale, there aren't many smooth surfaces in our bodies. Cells are covered with bumpy molecules that help them recognise each other and stick together. Between the cells the extracellular matrix – a mesh of proteins, carbohydrates, and other molecules – helps migrating cells find their destination. Smooth, metal hip implants don't look anything like biological surfaces. Many companies now coat bone implants with nanoscale-textured hydroxyapatite, a mineral found in bone. This hydroxyapatite coating tricks the body into incorporating the implant as though it was a real bone.
Encouraging bone growth
Hydroxyapatite coating can make the implants "stickier", but to have a truly successful implant, the surrounding normal bone needs to grow around the implant. Titanium nanotubes, built to resemble the proteins that our bodies use to stick cells together, could encourage this kind of integration. In experimental models they encourage the growth of osteoblasts, the cells that synthesise bone. If osteoblasts grow around the new implant, they could produce new bone all around it. Titanium nanotubes are being developed by a number of groups, and could be used in future dental and orthopaedic implants. Researchers are also trying to embed drugs that encourage bone growth into hydroxyapatite coatings.
The antimicrobial properties of nanosilver have been discussed in a previous post, and post-operation bacterial infections are a serious and common problem in orthopaedics. Nanosilver is used in bandages and other wound-healing materials, and is being investigated for potential use on the surfaces of orthopaedic implants. One potential problem is that silver nanoparticles also inhibit the growth of osteoblasts, so fighting infection and encouraging bone growth might not be simultaneously achievable with silver. Other, more creative solutions are needed.
Companies such as Amedica coat implants with silicon nitride to simultaneously decrease bacterial growth and encourage the formation of bone. Unlike silver, silicon nitride seems to be able to do both at the same time. This could be because at the nanoscale the silicon nitride is textured in a way that attracts osteoblasts and repels bacteria.
Thomas Webster is one of medical nanotechnology's pioneers. NanoShield, one of the nine start-up companies that has sprung from Dr Webster's work, is developing a nanosensor that can measure how well an implant is doing. Carbon nanotubes on the implant detect what kind of cells are attached to the implant, and transmit this information through an embedded microchip. Each cell in the body has different electrical properties, and these properties can tell the nanosensor if an osteoblast, an inflammatory cell, or a bacterium is attached. A nanostructured film on the implant could then release drugs, such as antimicrobials or anti-inflammatory molecules, depending on which type of cell is detected by the nanosensor.
Altering the surface of orthopaedic implants with nanotechnology has already improved the kinds of fake bones patients receive today. Further trickery will undoubtedly make them even better, and convince our own bones to grow around the imposter implant.